Indium tin oxide (ITO) and fluorine-doped tin oxide (FTO or SnO:F) coatings are widely used as window electrodes in opto-electronic devices. These transparent conductive oxides (TCOs) have been immensely successful in a variety of applications. Unfortunately, however, the use of ITO and FTO is becoming increasingly problematic for a number of reasons. Such problems include, for example, the fact that there is a limited amount of the element indium available on Earth, the instability of the TCOs in the presence of acids or bases, their susceptibility to ion diffusion from ion conducting layers, their limited transparency in the near infrared region (e.g., the power-rich spectrum that may benefit some photovoltaic devices), high leakage current of FTO devices caused by FTO structure defects, etc. The brittle nature of ITO and its high deposition and/or processing temperature(s) can also limit its applications. In addition, surface asperities in SnO2:F may cause problematic arcing in some applications.
Thus, it will be appreciated that there is a need in the art for smooth and patternable electrode materials with good stability, high transparency, and excellent conductivity.
The search for novel electrode materials with good stability, high transparency, and excellent conductivity is ongoing. One aspect of this search involves identifying viable alternatives to such conventional TCOs. In this regard, the inventor of the instant invention has developed a viable transparent conductive coating (TCC) based on carbon and, more specifically, based on graphene.
The term graphene generally refers to one or more atomic layers of graphite, e.g., with a single graphene layer or SGL being extendible up to n-layers of graphite (e.g., where n can be as high as about 10, preferably about 5). Graphene's recent discovery and isolation (by cleaving crystalline graphite) at the University of Manchester comes at a time when the trend in electronics is to reduce the dimensions of the circuit elements to the nanometer scale. In this respect, graphene has unexpectedly led to a new world of unique opto-electronic properties, not encountered in standard electronic materials. This emerges from the linear dispersion relation (E vs. k), which gives rise to charge carriers in graphene having a zero rest mass and behaving like relativistic particles. The relativistic-like behavior of delocalized electrons moving around carbon atoms results from their interaction with the periodic potential of graphene's honeycomb lattice and gives rise to new quasi-particles that at low energies (E<1.2 eV) that are accurately described by the (2+1)-dimensional Dirac equation with an effective speed of light νF≈c/300=106 ms−1. Therefore, the well established techniques of quantum electrodynamics (QED) (which deals with photons) can be brought to bear in the study of graphene—with a further advantageous aspect being that such effects are amplified in graphene by a factor of 300. For example, the universal coupling constant α is nearly 2 in graphene compared to 1/137 in vacuum. Moreover, it has been shown that graphene does not have any electronic band gap, which could open the door to novel opto-electronic applications.
Despite being only one-atom thick (at a minimum), graphene is chemically and thermally stable (although graphene may sometimes be surface-oxidized at 300 degrees C.), thereby allowing successfully fabricated graphene-based devices to withstand ambient and potentially harsh conditions. High quality graphene sheets were first made by micro-mechanical cleavage of bulk graphite. The same technique is being fine-tuned to currently provide high-quality graphene crystallites up to 100 μm2 in size. This size is sufficient for most research purposes in the micro-electronics field. Consequently, most techniques developed so far, mainly at universities, have focused more on the microscopic sample, and similarly have focused generally on device preparation and characterization rather than scaling up.
Unlike many current research trends, to realize the full potential of graphene as a possible TCC, large-area deposition of high quality material on substrates (e.g., silicon, glass, or plastic substrates, including coated versions of the same) is essential. To date, chemical vapor deposition (CVD) is seen by some as being the most promising process for the industrially viable large area growth of graphene. The accepted mechanism involves three steps, namely: (i) dissociation of the carbon precursor at high temperature (e.g., greater than 850 degrees C.) onto a polycrystalline metallic catalyst; (ii) carbon dissolution into the catalyst sub-surface; and (iii) graphene precipitation at the surface of the catalyst as the sample cools down.
Unfortunately, however, these techniques involve several drawbacks. First, they involve very high temperatures (e.g., greater than 850 degrees C. and sometimes higher than 950 degrees C.), as graphene quality is generally poor at lower temperatures since an amorphous graphitic carbon phase is always present given that the duration of the process is at least 30 minutes. Second, these techniques currently involve chemical etching of the catalyst for lift-off and transfer of the graphene onto the intended substrate. This process usually creases as well as contaminates the graphene film and, in general, is not scalable. The polycrystalline nature of the thick Ni, as well as its finite surface roughness, produces non-contiguous graphene domains of varying thickness (e.g., varying integer values of single layer graphene). This non-isotropic growth can be problematic for successful transfer and the fabrication of field effect devices based on graphene. Another characteristic of the incumbent process is that the catalyst film is a blanket film. But lift-off of a patterned thin film oftentimes causes the graphene to float and twist, making the transfer impractical.
Thus, it will be appreciated that it would be desirable to provide improved graphene-forming techniques, in terms of both scale and quality.
Certain example embodiments relate to a thermal annealing alternative to the above-mentioned deposition process, whereby precipitation of pristine graphene takes place directly onto the glass substrate at lower temperatures via a thin Ni metal or Ni alloy catalyst film pre-coated onto the intended glass substrate. Although the technique works well with MSVD deposited Ni thin films, it was found that a thin layer of ultra-smooth a-Ni provides a yet higher quality graphene (based on Raman data to date). The amorphous layer of Ni with no grain boundaries advantageously allows the graphene to precipitate in a more isotropic manner. It also was found that c-Ni and other Ni morphologies where there are numerous grain boundaries helps in the formation of high quality graphene. And to date, the graphene has been found to be very uniform over several tens of microns in length and width.
As explained in greater detail below, asymmetric growth of carbon occurring on both the gas-exposed and supporting sides of the Ni film was studied by in situ Raman Spectroscopy and differential scanning calorimetry (DSC). It may in certain example embodiments be possible to further relate the process conditions to the graphene growth at both the supporting and gas-exposed sides of the catalyst interface. Concepts of surface thermodynamics show that one driving force for such a growth is the gradient in the concentration of dissolved carbon between the gas-exposed and supporting sides. This surprisingly and unexpectedly gives rise to carbon diffusion flux through the catalyst.
Certain example embodiments of this invention relate to a method of making a coated article including a graphene-inclusive film on a substrate. A metal-inclusive catalyst layer (e.g., of or including Ni and/or the like) is disposed on the substrate. The substrate with the metal-inclusive catalyst layer thereon is exposed to a precursor gas (e.g., of or including acetylene) and a strain-inducing gas (e.g., of or including He) at a temperature of no more than 900 degrees C. (preferably no more than 800 degrees C., and still more preferably no more than 700 degrees C.—and for example, from 700-900 degrees C.). The strain-inducing gas induces strain in the metal-inclusive catalyst layer. Graphene is formed and/or allowed to form both over and contacting the metal-inclusive catalyst layer, and between the substrate and the metal-inclusive catalyst layer, in making the coated article. The metal-inclusive catalyst layer, as well as with graphene formed thereon, is removed, e.g., through excessive strain introduced into the catalyst layer as associated with the graphene formation (e.g., from the He-inclusive gas environment).
Certain example embodiments of this invention relate to a method of making a coated article including a graphene-inclusive film on a substrate. A metal-inclusive catalyst layer is disposed on the substrate. The substrate with the metal-inclusive catalyst layer thereon is rapidly heated (e.g., preferably within 1 minute, more preferably within 30 seconds, and possibly within about 10 seconds) to 700-900 degrees C. The substrate with the metal-inclusive catalyst layer thereon is annealed in a He gas inclusive environment (e.g., preferably for no more than 10 minutes, more preferably for no more than 7 minutes, and possibly for about 5 minutes), and the He gas is provided at a pressure selected to engineer a desired stress in the metal-inclusive catalyst layer. The substrate with the catalyst layer thereon is exposed to a carbon-inclusive precursor gas (e.g., preferably for no more than 5 minutes, more preferably for no more than 3 minutes, and possibly for about 20 seconds to 2 minutes). Graphene is formed and/or allowed to form both over and contacting the metal-inclusive catalyst layer, and between the substrate and the metal-inclusive catalyst layer, in making the coated article. The He gas may induce strain in the metal-inclusive catalyst layer sufficient to cause at least partial separation between the substrate and the metal-inclusive catalyst layer during graphene formation, and/or the metal-inclusive catalyst layer and the graphene thereon may be delaminated through excessive strain provided by the He gas.
Certain example embodiments relate to a method of making a coated article comprising a graphene-inclusive film on a substrate is provided. A metal-inclusive catalyst layer is disposed on the substrate. The substrate with the metal-inclusive catalyst layer thereon is heated to 700-900 degrees C. The substrate with the catalyst layer thereon is exposed to a carbon-inclusive precursor gas. Graphene is formed and/or allowed to form both over and contacting the metal-inclusive catalyst layer, and between the substrate and the metal-inclusive catalyst layer. The metal-inclusive catalyst layer and the graphene on the metal-inclusive catalyst layer are mechanically delaminated from the substrate so that the graphene formed between the substrate and the metal-inclusive catalyst layer remains on the substrate following the mechanical delaminating, in making the coated article (e.g., using an adhesive such as tape or the like). The metal-inclusive catalyst layer is engineered to have a stress that facilitates the mechanical delaminating (e.g., using a gas such as He and/or the like, and/or through other means).
According to certain example embodiments, rapid heating may be performed such that growth temperatures of 800-900 degrees C. are reached at the surface of the Ni and/or the surface of the substrate within 10 seconds, although other temperature ranges (e.g., from above Tg and around 600-900 degrees C., for example) also are possible. The heating may be performed at atmospheric pressure, a pressure less than atmospheric (e.g., in the presence of inert gas—and possibly at 0.5-10 Torr, more preferably 1-5 Torr, and sometimes about 2 Torr), in some example instances. This annealing may be performed very quickly, e.g., such that carbon is supplied for less than 10 minutes, more preferably less than 5 minutes, and still more preferably from about 20 seconds to 2 minutes, in certain example embodiments. Cooling also may be rapid in some cases, e.g., with the substrate being cooled at a rate of 5-20 degrees C. per second, more preferably 10-15 degrees C. per second, and sometimes about 13 degrees C. per second.
According to certain example embodiments, the catalyst layer may comprise Ni metal, a-Ni, a-Ni:P, c-Ni, and/or the like.
According to certain example embodiments, the substrate with the catalyst layer thereon may be exposed to at least helium and/or acetylene gasses, in plural successive stages. For instance, a first stage may comprise providing at least helium gas at a first flow rate, and a second stage may comprise providing at least helium gas at a second flow rate and acetylene gas at a third flow rate, with the first and second stages being provided in that order. The first flow rate may be greater than the second and third flow rates, and the second flow rate may be lower than the third flow rate. No or virtually no acetylene may be provided in the first stage in some example instances. In an optional third stage that follows the second stage, no or virtually no helium and/or acetylene is provided. Although a statement is made that no or virtually no gas is provided in some cases, it will be appreciated that some gas(es) may be provided unintentionally, e.g., as a substrate moves through the successive stages, as a result of normal manufacturing processes. Oxygen preferably is not involved in this process. The temperature may be significantly reduced over the course of optional third stage in certain example embodiments.
According to certain example embodiments, the catalyst layer may be patterned to a desired pattern (e.g., via photolithography and masking with a photoresist or the like, laser ablation/etching, ion beam milling, and/or the like). The graphene-inclusive film, once formed on the coated article may generally correspond to the desired pattern (e.g., by virtue of it being formed in connection with (i.e., on and/or under) the patterned catalyst layer). In other cases, the graphene-inclusive film may be blanket coated directly or indirectly on a substrate.
According to certain example embodiments, the metal-inclusive catalyst may have a smoothness that is at least as smooth of its underlying substrate. In some cases, the metal-inclusive catalyst may have a smoothness that is on the order of glass.
According to certain example embodiments, graphene may be formed directly or indirectly on a thin film layer such as, for example, a metal. In some instances, this arrangement may provide anticorrosion, mechanical durability, and/or the like.
Articles made using these methods and products incorporating such articles also are contemplated herein. Windows, photovoltaic devices, displays, etc., are example applications that may benefit from the technology disclosed herein. In general, the techniques disclosed herein may be used anywhere a TCC would be desirable.
The features, aspects, advantages, and example embodiments described herein may be combined to realize yet further embodiments.